1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor which detects change in the magnetic permeability of a magnetostrictive material and outputs the change in the form of electrical change, thereby detecting torque applied to the rotating shaft, and an electric power steering apparatus including this magnetostrictive torque sensor mounted therein.
2. Description of the Related Art
Magnetostrictive torque sensors are mounted in electric power steering apparatuses for vehicles. The electric power steering apparatus is a support apparatus which, when a driver turns the steering wheel while driving the vehicle, has a motor cooperate therewith so as to assist the steering effort. In the electric power steering apparatus, the magnetostrictive torque sensor detects the steering torque caused in the steering shaft coupled to the steering wheel by the driver turning the steering wheel. The electric power steering apparatus controls the auxiliary steering force that is the output from the motor based on at least the detected steering torque and a vehicle speed signal from a vehicle speed sensor, which detects the speed of the vehicle, thereby reducing the steering effort of the driver.
A magnetostrictive torque sensor has been proposed where two magnetostrictive materials are provided one above the other on the surface of the rotating shaft that is the steering shaft so as to have opposite directions of magnetic anisotropy (refer to, e.g., JP2004-245636A). When the steering torque acts on the rotating shaft, rotation moment is produced and the rotating shaft is twisted, which distorts the magnetostrictive materials, and one of the magnetostrictive materials is further distorted while the distortion of the other is decreased because of an inverse magnetostriction characteristic due to magnetic anisotropy. Such increase and decrease in distortion change the alternating-current resistance and the like of detection coils provided around the magnetostrictive materials. The magnetostrictive torque sensor detects this change, thereby detecting the steering torque acting on the rotating shaft.
When the auxiliary steering force from the motor is transmitted to the rotating shaft via gears, bending moment to bend the center axis of the rotating shaft as well as rotation moment around the axis acts on the rotating shaft. This bending moment varies in magnitude according to the position of a section along the axis direction of the rotating shaft. To be specific, the bending moment is at zero at its point of action and increases as the section position goes away from the point of action. Hence, bending moments different in magnitude act respectively on the two magnetostrictive materials different in the position where they are formed. For the two magnetostrictive materials, the rotation moment and the bending moment are both detected as the distortion thereof, causing the measurement to deviate from a true value, and hence the accuracy of the magnetostrictive torque sensor for detecting rotation moment decreases. This is because bending moments different in magnitude act on the two magnetostrictive materials, and thus the bending moment components cannot be cancelled although a difference is obtained in output such as the alternating-current resistance from the respective detection coils.
Accordingly, JP2004-245636A further discloses a magnetostrictive torque sensor where one end of the rotating shaft is an open end so that no bending moment acts on the two magnetostrictive materials.
However, in such a magnetostrictive torque sensor, one end of the rotating shaft cannot be made completely open, but the one end is coupled to the steering wheel, so that when strongly turning the steering wheel as in performing stationary steering, e.g., while driving the vehicle into the garage, bending moment from the steering wheel may act on the rotating shaft. Further, it may be demanded that both ends of the rotating shaft be rotatably fixed without leaving one end open so that the rotating shaft is rotatably, stably fixed, depending on the environment in which the magnetostrictive torque sensor is installed.